Cracking furnaces, the primary equipment in the petrochemical industry, are mainly used for heating hydrocarbon material so as to achieve cracking reaction which requires a large amount of heat. Fourier's theorem says,
      q    A    =            -      k        ⁢                  d        ⁢                                  ⁢        t                    d        ⁢                                  ⁢        y            wherein q is the heat transferred, A represents the heat transfer area, k stands for the heat transfer coefficient, and dt/dy is the temperature gradient. Taking a cracking furnace used in the petrochemical industry as an example, when the heat transfer area A (which is determined by the capacity of the cracking furnace) and the temperature gradient dt/dy (which is determined by the furnace coil material and burner capacity) are determined, the only way to improve the heat transferred per unit area q/A is to improve the value of the heat transfer coefficient k, which is subject to influences from thermal resistance of the main fluid, thermal resistance of the boundary layer, etc.
In accordance with Prandtl's boundary layer theory, when an actual fluid flows along a solid wall, an extremely thin layer of fluid close to the wall surface would be attached to the wall without slippage. That is to say, the speed of the fluid attached to the wall surface, which forms a boundary layer, is zero. Although this boundary layer is very thin, the heat resistance thereof is unusually large. When heat passes through the boundary layer, it can be rapidly transferred to the main fluid. Therefore, if the boundary layer can be somehow thinned, the heat transferred would be effectively increased.
In the prior art, the furnace pipe of a commonly used cracking furnace in the petrochemical industry is usually structured as follows. On the one hand, a rib is provided on the inner surface of one or more or all of the regions from the inlet end to the outlet end along the axial direction of the furnace coil in the cracking furnace, and extends spirally on the inner surface of the furnace coil along an axial direction thereof. Although the rib can achieve the purpose of agitating the fluid so as to minimize the thickness of the boundary layer, the coke formed on the inner surface thereof would continuously weaken the role of the rib as time lapses, so that the function of reducing the boundary layer thereof will become smaller. On the other hand, a plurality of fins spaced from one another are provided on the inner surface of the furnace pipe. These fins can also reduce the thickness of the boundary layer. However, as the coke on the inner surface of the furnace pipe is increased, these fins will similarly get less effective.
Therefore, it is important in this technical field to enhance heat transfer elements so as to further improve heat transfer effect of the furnace coil.